Orthogonal frequency division multiplexing (OFDM) modulation has found widespread acceptance in many wireless communication systems, such as Wireless Local Area Network (WLAN) architectures, Digital Audio/Video Broadcasting (DAB/DVB) transmission, and mobile radios. An OFDM-based radio distributes data over a large number of modulated subcarriers spaced apart at precise frequencies, as illustrated by the gain-frequency plots in FIG. 9. FIG. 9 depicts an OFDM, unmodulated subcarrier (top plot) and the same subcarrier with OFDM and 64-state quadrature amplitude modulation (QAM) as specified by IEEE 802.11a/g (bottom plot).
In general, OFDM provides high data throughput and high spectral efficiency multiple access to a communications channel with low multi-path and/or delay spread distortion. However, OFDM also requires high Peak-to-Average Power Ratio (PAPR), is sensitive to Inter-Modulation Distortion (IMD) and local oscillator (LO) phase noise, and requires linear and wide bandwidth transceiver hardware. Thus testing and corrective measures are required during initial design installation or troubleshooting of an impaired OFDM transceiver.
Conventional measuring methods for OFDM-based transceivers each have disadvantages. The single-tone method is slow and requires iterative tests to cover the full bandwidth, and does not measure inter-modulation distortion (IMD). Test conditions in the multi-tone method may not match the actual application, an important consideration, and ad hoc test frequency selection leads to difficulties in comparing test results. The impulse/step response method requires high signal peak-to-average ratio for simple test vectors and extensive post-processing for complex test vectors, but provides only poor frequency resolution and does not pinpoint sources of impairment. Measurement of the error vector magnitude (EVM) involves high setup costs for the requisite customized equipment, is slow, requiring software post-processing, and does not pinpoint sources of impairment. Measurement of bit error rate (BER) requires slow and time-consuming iterative tests and does not pinpoint sources of impairment, with packet, frame, or block error rate testing having similar disadvantages.
There is, therefore, a need in the art for an improved measurement technique for multi-subcarrier wireless systems.